Alloy steel and its preparation



United States Patent "ice 3,392,012 ALLOY STEEL AND ITS PRElARATION Lee S. Richardson, West Chester, Pa., assignor to Foote Mineral Company, Exton, Pin, a corporation of Pennsylvania No Drawing. Filed June 4, 1965, Ser. No. 461,517 11 Claims. (Cl. 75-123) ABSTRACT (3F THE DISCLOSURE A maraging steel having strength, toughness and ductility comparable to that of 1 nickel maraging steel is provided containing 1114% nickel and 1.5-3% rnanganese along with 69% cobalt and 3-5 molybdenum, and, in a preferred embodiment, up to 0.4% of vanadium.

The present invention relates to a novel alloy steel possessing very high strength coupled with toughness and ductility and to its preparation; and, more particularly, the present invention relates to an age-hardened martensitic steel alloy having a strength, toughness and ductil ity comparable to that of the 18% nickel maraging steel but less expensive.

There is a class of ferrous-base alloys containing nickel with interrelated amounts of cobalt and molybdenum which can be age-hardened while in the martensitic condition. These alloys are known as 'maraging steels and are characterized by very high strength coupled with high toughness and ductility. The common commercial variety of these alloys is the so-called 18% nickel maraging steel which contains 17l9% nickel, 79% cobalt, 3-5 molybdenum, (HS-0.8% titanium, 0.1% aluminum, less than 0.03% carbon, less than 0.1% manga nese, less than 0.1% silicon and the balance essentially iron. This steel has an ultimate tensile strength from 200,000 to 300,000 p.s.i., depending upon exact composition; a high toughness, as shown by the ratio of notch tensile strength to ultimate tensile strength of 1.3-1.6; good ductility, as shown by an elongation (in 2 inches using a Mi" diameter tensile test bar) of 68%, and a reduction in area of 40-65%.

Basic work on such alloys is reported in an article entitled 18% Nickel Maraging Steel by R. F. Decker, et al., in Transactions of the ASM, vol. 55, pp. 5876 (March 1962). These authors state that manganese is detrimental to the alloys and show that increasing the manganese content from 0.07% to 1.9% lowers the notch tensile strength from 350,000 to 178,000 psi. and the notch tensile strength: ultimate tensile strength ratio from 1.32 to 0.62.

United States Patent 3,093,519 issued to the authors of the foregoing paper and reflects work on which the paper is based. This patent also states that manganese is undesirable in the alloys and that ordinarily it is advantageous to maintain the manganese content below about 1% and even below about 0.25% in order to assure that the alloys have adequate toughness. However, the patent also states that under undisclosed special circumstances manganese can be present in amounts up to United States Patents 3,132,937 and 3,132,938 disclose improvements in the maraging steel of Patent 3,093,519. Each of these patents states that the alloys may contain up to about 0.2% manganese.

Contrary to the teachings of the foregoing prior art, it has been discovered that a substantial amount of manganese is not detrimental or undesirable in certain nickel- Patented July 9, 1968 cobalt-molybdenum alloy steels but in fact can be substituted for nickel in certain amounts to provide an agehardened martensitic alloy having a strength, toughness and ductility comparable to the above-described 18% nickel maraging steelprovided certain critical requirements, particularly with respect to content of nickel, cobalt, molybdenum and manganese, and other constituents, are observed.

It is, therefore, the principal object of the present invention to provide a novel nickel-containing ferrous base alloy of the maraging type in which a portion of the nickel is replaced by manganese.

It is another object of the present invention to provide a novel maraging steel alloy having strength, toughness and ductility comparable to conventional 18% nickel maraging steel but less expensive to manufacture by virtue of the replacement of a portion of the nickel by manganese.

It is a further object of the present invention to provide a method for manufacturing the novel alloy steel.

Other objects will become apparent from a consideration of the following specification and claims.

The novel age-hardened, martensitic alloy steel of the present invention consists essentially of, in percent by weight, from 11 to 14% of nickel, from 6 to 9% of cobalt, from 3 to 5% of molybdenum, from 1.5 to 3% of manganese, no more than about 0.2% of aluminum, no more than 0.4% of titanium, carbon in an amount less than 0.03%, less than 0.15% of silicon and the balance essentially iron. In the embodiment where manganese is present in amounts between 2.5 and 3%, vanadium is also included in an amount up to 0.4% and this provides a further surprising increase in notch tensile strength.

The foregoing age-hardened, martensitic alloy steel possesses strength coupled with toughness and ductility comparable to those of presently available 18% nickel maraging steel. Yet, because of the replacement of a sub stantial portion of the nickel, present in the 18% nickel product, with manganese the present alloy steel can be prepared at less cost than the conventional 18% nickel maraging steel. The properties possessed by the present product are surprising in view of the beliefs prevailing based on the prior work referred to above.

The present alloy may be prepared in the same manner as conventional 18% nickel maraging steel with the exception, however, that manganese will be added to provide the defined manganese content (with a corresponding reduction in nickel to within the defined range of nickel content). Advantageously, at least the principal source of manganese, and preferably the entire source, will be electrolytic manganese. Likewise in selecting sources of the other constituents care will be observed in selecting those sufficiently low in carbon and also in silicon. Electrolytic iron and sponge iron may be used as the source of iron. Electrolytic nickel and electrolytic cobalt may be the sources of these elements. Low carbon ferromolybdenu'm may be the source of molybdenum and part of the iron. Titanium, it included, may be pro vided by high purity titanium metal or by low-carbon ferrotitanium; aluminum, if included, may be provided by commercially pure aluminum, and vanadium, when included, may be provided by low carbon ferrovanadium. -t will be noted that the present alloy steel is essentially free of chromium.

The alloy steel is prepared by first melting the constituents, as in a high frequency induction furnace. While this may be done in air, it is preferably accomplished under an inert atmosphere, including a vacuum. Although, if the carbon content is too high it may be reduced by oxidation followed by deoxidation with, for example, carbon monoxide, aluminum or titanium, it is preferred to use starting materials sufiiciently low in carbon such that oxidation and deoxidation are not required.

The molten material is then cast in a mold of, for

example, sand, ceramic or metal, like cast iron.

The cast material is homogenized thermally at a temperature between about 1400 F. and about 1800 F. and then allowed to air cool to room temperature to convert the substantially homogeneous austenitic facecentered cubic structure to a homogeneous body-centered cubic structure by means of a martensitic transformation. This solution annealing and martensitic transformation may be, and preferably is, preceded by a hot working treatment at from about 1800 F. to about 2300 F. during which working one or more of the dimensions of the casting is reduced.

The martensitic alloy is then subjected to age-hardening (maraging) by heating to between about 750 and about 1000 F. for from about 1 to about hours.

As will be seen from the following examples and tables, age-hardened, martensitic alloy steels prepared according to the present invention possess strength, toughness and ductility comparable to those of conventional 18% nickel maraging steel. The following examples are given for the purpose of illustration and are not intended to limit the scope of the invention in any way.

Examples The alloys set forth in the following tables were prepared as follows: Electrolytic iron, electrolytic nickel, electrolytic cobalt and low carbon ferromolybdenum (60% molybdenum and less than 0.02% carbon) are melted together under vacuum in a magnesia crucible held in a 9,600 cycle induction furnace. In those examples where vanadium is included, low-carbon ferrovanadium (90% vanadium and less than 0.06% carbon) is part of the melt. The furnace is then filled with argon at one atmosphere, and electrolytic manganese, high purity titanium and commercially pure aluminum are added and melted in. The melt is then cast in a cast iron mold as a lb. ingot having cross-sectional dimensions of approximately 4" x 4". The ingot is then hot forged into bars having cross-sectional dimensions of approximately 1" x 1", and rough machined into rough test bar blanks. These are then solution annealed for 1 hour at 1500 F., and allowed to cool to room temperature at the rate of about 10 F. per minute. The test bar blanks are then age-hardened at 900 F. for 3 hours and allowed to cool to room temperature at the rate of about 10 F. per minute. The blanks are then machined to the final tolerances for testing, and, in the case of the bars used for measuring notch tensile strength, an annular notch is cut.

Strength, in terms of ultimate tensile strength (UTS) is measured by ASTM Standard E857T, Tension Testing of Metallic Materials (A 370-54T, Mechanical Testing of Steel Products). Toughness is indicated by the ratio of notch tensile strength (NTS) to ultimate tensile strength, notch tensile strength being measured according to Materials Research and Standards, March 1962, pp. 196203, using a major diameter of 0.252, a minor diameter of 0.177" and a notch radius of about 0.001". Ductility is determined by the percent elongation in two inches on a 0.25 diameter test bar and the percent reduc tion in area, both measured during the test for ultimate tensile strength referred to above.

In all the following alloys of the invention, the amounts of nickel, manganese, cobalt, molybdenum, titaniurn, vanadium, silicon and carbon are as set forth in the tables (in terms of percent by weight by chemical analysis), the aluminum is 0.1% (added), and the balance is essentially iron.

TABLE A Example 1 2 3 4 5 6 7 8 Ni 14.0 13.6 12.8 12.6 12.6 11.7 11.8 11.3 8.2 6.0 8.3 8.6 7.1 7.2 8.9 6.4 3.9 4.0 4.2 4.1 4.0 3.1 4.9 3.0 1.6 2.0 1.75 1.7 2.0 2.0 2.0 1.8 0 0. 08 0 0 0 0 0 0 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 .15 0.16 0.18 0.17 0.16 0.16 0. 20 0.16 .02 .02 02 02 02 02 .02 .02 Si 0.01 0.01 0.01 0.04 .01 .01 01 .01 UTS (thousand p.s.i 2 228 236 227 231 217 248 201 NTS (thousand p.s.i. 2 310 348 344 324 274 338 272 Percent elongation in 2 inches 6.5 7 6.5 7 5.5 6 5 7 7 Percent reduction in area 52 60 56 60 44 58 52 60 NTS/UTS 1.44 1.36 1.47 1.52 1.40 1.33 1.36 1.35

UTS (thousand p.s.i.) NTS (thousand p.s.i.) 302 345 350 334 353 362 344 Percent elongation in 2 inches 6 6 5 7 6 4 6. 5 7 Percent reduction in area 52 54 58 43 40 54 57 NTS/UTS 1. 37 1. 49 1. 45 1. 25 1. 30 1. 43 1. 53

For comparison, the average results from tests of nine examples of 18% nickel maraging steel made using the same procedure as in the foregoing examples and containing about 17-18% nickel, about 79% cobalt, about 35% molybdenum and 0.12-0.19% titanium are as follows: Y

UTS p.s.i 201,000 NTS p.s.i 311,000 NT S/UTS 1.55 Percent elongation in 2 inches 7.5 Percent reduction in area 62 The alloy steels of the present invention have a UTS of at least about 180,000 p.s.i., preferably at least about 235,000 p.s.i. and up to about 270,000 p.s.i.; an NTS of at least about 270,000 p.s.i., preferably at least about- 345,000 p.s.i., and up to about 360,000 p.s.i.; an NTS/ UTS ratio of at least about 1.25, preferably at least about 1.40, and up to about 1.55; :an elongation in 2" of at least about 4%, preferably at least about 5%, and up to about 8%; and a reduction in area of at least about 40%, preferably at least about 45%, and up to about 60%.

The preferred composition of the present alloy steel is 12-13% nickel; about 8% cobalt; about 4% molybdenum; 1.52.5% manganese when vanadium is absent or 2.5- 3% manganese when vanadium is present at about 0.2%; about 0.1% aluminum; 0.150.2% titanium; carbon in an amount less than 0.03%; less than 0.15% silicon, and the balance essentially iron.

Modification is possible in the selection of constituents and amounts thereof as well as in the particular techniques employed in preparing the product Without departing from the scope of the invention.

What is claimed is:

1. An age-hardened, martensitic iron-base alloy consisting essentially of, in percent by weight, from 11 to 14% of nickel, from 6 to 9% of cobalt, from 3 to 5% of molybdenum, from 1.5 to 3% of manganese, no more than about 0.2% of aluminum, no more than 0.4% of titanium, carbon in an amount less than 0.03%, less than 0.15 of silicon and the balance essentially iron.

ate-2,012

2. The alloy of claim 1 having an ultimate tensile strength of at least about 180,000 p.s.i., a notch tensile strength of at least about 270,000 p.s.i., a ratio of notch tensile strength to ulirnate tensile strength of at least about 1.25, an elongation in 2 inches of at least about 4%, and a reduction in area of at least about 40%.

3. The alloy of claim 1 wherein the manganese is present in an amount from 2.5 to 3%, and wherein vanadium is present up to 4.0%.

4. The alloy of claim 1 wherein vanadium is present up to 0.4%.

5. The alloy of claim 4 wherein the alloy contains at least 2% of manganese.

6. The alloy of claim 3 having an ultimate tensile strength of at least about 180,000 p.s.i., a notch tensile strength of at least about 270,000 p.s.i., a ratio of notch tensile strength to ultimate tensile strength of at least about 1.25, an elongation in 2 inches of at least about 4%,

and a reduction in area of at least about 40%.

7. An age-hardened, martensitic iron-base alloy consisting essentially of, in percent by weight, l2-13% of nickel, about 8% of cobalt, about 4% of molybdenum, l.52.5% manganese, about 0.1% of aluminum, 0.15- 0.2% of titanium, carbon in an amount less than 0.03%,

less than 0.15% of silicon, and the balance essentially iron.

8. The alloy of claim 7 having an ultimate tensile strength of at least about 235,000 p.s.i., a notch tensile strength of at least about 345,000 p.s.i., a ratio of notch tensile strength to ultimate tensile strength of at least 6 about 1.40, an elongation in 2 inches of at least about 5%, and a reduction in area of at least about 45%.

9. The alloy of claim 8 wherein the manganese con tent is about 2% and wherein vanadium is present up to 0.4%.

10. An age-hardened, martensitic iron-base alloy consisting essentially of, in percent by weight, 12-13% of nickel, about 8% of cobalt, about 4% of molybdenum, 2.53% of manganese, about 0.2% of vanadium, about 0.1% of aluminum, (MS-0.2% of titanium, carbon in an amount less than 0.03%, less than 0.15% of si.icon, and the balance essentially iron.

11. The alloy of claim 10 having an ultimate tensile strength of at least about 235,000 p.s.i., a notch tensile strength of at least about 345,000 p.s.i., a ratio of notch tensile strength to ultimate tensile strength of a least about 1.40, an elongation in 2 inches of at least about 5%, and a reduction in area of at least about 45%.

References Cited UNITED STATES PATENTS 3,093,519 6/1963 Decker et al 75-l23 XR 3,243,285 3/1966 Fragetta et a1. 75l23 3,318,690 5/1967 Floreen et a1. 75123 HYLAND BlZOT, Primary Examiner.

DAVID L. BECK, Examiner.

P. WEINSTEIN, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,392,012 July 9, 1968 Lee S. Richardson It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4, TABLE B, fifth and sixth columns, line 9 thereof, .O2", each occurrence, should read .0l Column 5, line 9, "4.0%" should read 0.4%

Signed and sealed this 16th day of December 1969.

(SEAL) Attest:

Edward M. Fletcher, Jr.

Jitiesting Officer Commissioner of Patents 

